Hollow fiber membranes can be made from a variety of polymers by different methods. One method involves non-solvent induced phase separation (NIPS) as taught for example in U.S. Pat. Nos. 3,615,024; 5,066,401; and, 6,024,872. Another method involves thermally induced phase separation (TIPS) as taught for example in U.S. Pat. Nos. 4,702,836 and 7,247,238. The membranes may have separation layers on their inner surfaces or their outer surfaces and may be used, for example, for microfiltration (MF) or ultrafiltration (UF).
The strength of a hollow fiber membrane can be increased by coating a membrane dope on a pre-formed tubular braid. U.S. Pat. Nos. 5,472,607 and 6,354,444 to Mahendran et al. teach coating a membrane on the outside of a braid with limited penetration. U.S. Pat. No. 4,061,861 to Hayano et al., U.S. Pat. No. 7,267,872 to Lee et al., and U.S. Pat. No. 7,306,105 to Shinada et al. also teach braid supported membranes. Hollow fiber membranes made according to the teachings of Mahendran et al. have been successfully commercialized.
Another approach that has been proposed for making a reinforced hollow fiber membrane involves embedding fibers within the wall of a hollow fiber while the hollow fiber is being cast. US Publication 2002/0046970 to Murase et al., International Publication WO 03/097221 to Yoon et al. and U.S. Pat. No. 6,454,943 to Koenhen describe methods of embedding a monofilament or a multi-filament yarn longitudinally within the wall of a hollow fiber. None of these methods have produced commercially available membranes.
Recent International Publications WO 2010/108285 and WO 2010/148517 by the present inventors teach forming cage like reinforcing structures within the wall of a hollow fiber while the hollow fiber is being cast. Filaments of the reinforcing structures may be bonded together at points of contact, for example by heat, UV light, or solvent bonding through a component of the filaments adapted to the bonding method.